The present invention relates to a technique for forming a ferroelectric film on a substrate. More specifically, the present invention relates to a solution and a method for forming a uniform ferroelectric film free from uneven coating (striation) on a substrate.
Films of lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), strontium bismuth tantalate (SBT) and other composite oxides can be used for DRAMs, FRAMs and other semiconductor memory devices, as well as for capacitors, sensors, actuators, etc. because of their high dielectricity, ferroelectricity, piezoelectric effect and pyroelectricity.
Solutions prepared from an organic solvent and metal alkoxides having an element constituting the ferroelectric film or other organic metal compounds are used for forming ferroelectric films. Alcohol, ethylene glycol derivatives, xylene etc. can be used as an organic solvent for these solutions; but ethylene glycol derivatives, especially ethylene glycol monomethyl ether are most widely used (JP, 5-319958, A, JP, 7-90594, A).
However, in recent years, the harmfulness of ethylene glycol monomethyl ether and other ethylene glycol derivatives has been being considered to be a problem. Consequently, safety of ethylene glycol derivatives has been actively debated and legislation restricting their use has been introduced in some countries, since their adverse effect on the reproductive function has been discovered. In response to this international trend, in Japan too, there has been a movement, especially in the electronics industry, to restrict the use of ethylene glycol derivatives; hence developing solutions for forming ferroelectric films not employing ethylene glycol derivatives has been desired.
A solution and a method for forming ferroelectric films not using ethylene glycol derivatives are known from JP, 9-278415, A, and JP, 10-226519, A, wherein a low toxic organic solvent having little influence on the reproductive function such as primary alcohol, propylene glycol monomethyl ether or propylene glycol monoethyl ether is used.
However, when using these organic solvents for forming a film on a substrate, especially when the film is formed by the so-called spin coating method in which the substrate placed on a spinner is rotated at a high speed after a solution has been dripped onto it, striation tends to occur, so that it is difficult to form a uniform ferroelectric film.
It is clear that with a capacitor electrode area of several xcexcm or less, as used in actual electronic devices, striation, which is a stripe-shaped film thickness distribution, causes inhomogeneity of capacitor properties. Consequently, to eliminate such unevenness of device properties"" the problem of striation needs to be resolved.
Accordingly, it is the object of the present invention to solve the before-mentioned problems and to provide a low toxic solution and a method for forming a uniform ferroelectric film free from striation.
As a result of the extensive research to solve the above-mentioned problems, the inventors of the present invention have completed the invention by finding that these problems can be solved by including one or more members selected from the group consisting of modified silicone oil and a fluorinated surfactants in a solution for forming a ferroelectric film using a primary alcohol and/or propylene glycol monoalkyl ether as a solvent.
In other words, the present invention relates to a solution for forming a ferroelectric film prepared from primary alcohol and/or propylene glycol monoalkyl ether and organic metal compounds having an element constituting the ferroelectric film, comprising one or more members selected from the group consisting of modified silicone oil and fluorinated surfactants.
The present invention further relates to the before-mentioned solution, characterized in that the organic metal compounds are 1 or more members selected from the group consisting of lead compounds, zirconium compounds, titanium compounds and lanthanum compounds.
The present invention further relates to the before-mentioned solution, wherein the fluorinated surfactant is a nonionic or an anionic surfactant.
The present invention further relates to the before-mentioned solution, wherein the content of the modified silicone oil is 1 to 100 ppm.
The present invention further relates to the before-mentioned solution, wherein the content of the fluorinated surfactant is 10 ppm to 5%.
The present invention further relates to the before-mentioned solution, characterized in that the ferroelectric is lead zirconate titanate (PZT) or lead lanthanum zirconate titanate (PLZT).
The present invention also relates to a method for forming a ferroelectric film, wherein a solution for forming a ferroelectric film, prepared from primary alcohol and/or propylene glycol monoalkyl ether and organic metal compounds having an element constituting the ferroelectric film, said solution being obtained by including 1 or more members selected from the group consisting of modified silicone oil and fluorinated surfactants, is used for coating a substrate which is subsequently dried and baked.
The present invention further relates to the before-mentioned method, characterized in that the substrate is made from materials including platinum, iridium, iridium oxide, titan, titan oxide, strontium ruthenium oxide (SRO) or indium tin oxide (ITO).
The present invention further relates to the before-mentioned method, wherein the ferroelectric film is a lead zirconate titanate (PZT) and lead lanthanum zirconate titanate (PLZT) film.
The present invention further relates to the before-mentioned method, characterized in that the solution is coated onto the substrate by the spin coating method.
The present invention further relates to the before-mentioned method, characterized in that the drying and baking process consists of drying, prebaking and baking and in that the heating temperatures are higher than 100xc2x0 C., 300-650xc2x0 C. and 400-800xc2x0 C., respectively.
By means of the present invention, it is possible to provide a low toxic solution for forming a uniform ferroelectric film free from striation.
With the solution for forming a ferroelectric film obtained according to the present invention, it is also possible to form a uniform film free from striation by means of the spin coating method.
Further, the solution for forming a ferroelectric film prepared from primary alcohol and/or propylene glycol monoalkyl ether and organic metal compounds having an element constituting the ferroelectric film, signifies a solution comprising components of an organic metal compounds in an organic solvent, for example, a solution in which organic metal compounds have been dissolved in an organic solvent or a solution in which a reaction product has been dissolved in an organic medium, which reaction product can be produced by alcohol exchange reaction (a reaction between organic metal compounds and organic solvents), complex alkoxide forming reaction (a reaction between an alcohol exchange reaction product and an organic metal compound or a reaction between organic metal compounds) or coordination of an organic solvent to a metal atom, etc.